1. Field of the Invention
The present invention relates to an apparatus for heating intake air arranged in an intake air passage used in, for example, a diesel-powered automotive vehicle.
2. Description of the Related Art
As a prior art, an apparatus for heating intake air has been proposed in which a honeycomb structure of semiconductive ceramic material having a positive temperature coefficient is used for a heater member of the heating apparatus. Such a heater member is generally known as a positive temperature coefficient (PTC) heater, and has a self-control temperature characteristic. In this type of heater member a complicated control circuit is not necessary, and there is less possibility of any excessive rise in temperature, and accordingly, such a heater member is considered to be safe. An example of the above prior art apparatus for heating intake air is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-59130.
However, when the above heater member is assembled as a constituent of a heating apparatus, a problem may arise in that cracks can occur in the heater member if the heater member is abruptly heated, since the thermal expansion coefficients between the heater member and a metal frame member are different; that is, 8.times.10.sup.-6 /.degree.C. for the heater member and 2.5.times.10.sup.-5 /.degree.C. for the metal frame member. Another problem is that insufficient contact is realized between the heater member and an electrode lead member of an assembly because of the strength of the heater member, and thus abnormal heating may occur and cause the electrodes to melt.